A study will be made of the molecular basis for the differences in ease of tolerance induction amongst inbred strains of mice with immunoglobulin antigens with emphasis on the role of the macrophage. The azobenzenearsonate (ABA) hapten system will be used to explore the relation between antigen and Ia determinants on the macrophage in triggering T-cells. The cellular basis of T-cell - T-cell interaction in the induction of suppression to this determinant will be studied. The nature and specificity of receptors on helper T-cells, suppressor T-cells and B-cells specific for the ABA determinant will be examined.